1. Field of the Invention
The present invention relates to the field of electrochemical fuel cells such as proton exchange membrane (PEM) fuel cells with thermal and moisture management. More particularly, the present invention relates to fuel cell systems with evaporative cooling and methods for humidifying and adjusting the temperature of the reactant streams.
2. Discussion of the Background
Electrochemical fuel cells generate electrical energy by converting chemical energy derived from a fuel directly into electrical energy by oxidation of the fuel in the cell. A typical fuel cell includes an anode, a cathode and an electrolyte. The reactant streams as fuel and oxidant are supplied to the anode and cathode, respectively. In electrochemical fuel cells employing hydrogen as the fuel and oxygen containing gas as the oxidant, the reaction product is water. At the anode, the fuel permeates the electrode material and reacts at the anode catalyst layer to form cations, which migrate through the electrolyte to the cathode. At the cathode, the oxygen containing gas supply reacts at the cathode catalyst layer to form anions. The anions react with cations to form a reaction product. The fuel cell generates a useful electric current and the reaction product is removed from the cell. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream. The anions O2xe2x88x92 formed at the cathode react with hydrogen ions 2H2+ that have crossed the membrane to form liquid water as the reaction product.
Unfortunately, it is not only electricity and product water that are generated during this process but also heat. The heat is produced primarily at the cathode when the hydrogen and oxygen ions combine. Some of this heat (about one third or less) can be removed by conventional evaporation of this product water, but the remaining heat must be removed by other means.
There is also another problem for reliable operation of fuel cells. Hydrogen ion conductivity through ion exchange membranes generally requires the presence of water molecules. The fuel and oxidant gases (especially fuel) are humidified prior to introducing them to the cell to maintain the water saturation of the membranes within the membrane electrode assembly. Currently, the most popular, perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, must be hydrated or saturated with water molecules for ion transport to occur. It is well known that such perfluorosulfonic membranes transport cations using a xe2x80x9cwater pumpingxe2x80x9d phenomenon. Water pumping involves the transport of cations in conjunction with water molecules, resulting in a net flow of water from anode side of the membrane to the cathode side. Thus, membranes can dry out on the anode side if water molecules are not resupplied.
Fuel cells employing such membranes require water to be removed from the cathode side. There must not be so much water that electrodes, which are bonded to the electrolyte, flood and thereby block the pores in the electrodes or gas diffusion layer. A balance is therefore needed.
There are other important aspects of the methods of operation of fuel cell systems and fuel cell design. The most important considerations (especially for PEM fuel cells) are the method of cooling and adjusting the temperature of the reactant streams for the fuel cell, the process of humidifying the reactant streams, and the water management for the fuel cell.
Cooling for the fuel cell has been provided by reactants, natural convection, radiation, and possible supplemental cooling channels and/or cooling plates. The herein system uses an evaporative cooling process as the mechanism to provide cooling, either to coolants or to the reactant gases.
In this regard, U.S. Pat. No. 3,761,316 discloses a fuel cell with evaporative cooling. A fuel cell assembly utilizing the waste heat of a fuel cell to provide evaporative cooling of the cell is provided by a hydrophobic separator disposed in heat conducting relationship with the fuel cell. A coolant liquid (water) is fed under pressure to a cavity on one side of the hydrophobic separator, and as vapor evolves from the coolant liquid, it passes through the hydrophobic separator to ambient.
In U.S. Pat. No. 4,795,683, a method of evaporative cooling a PEM fuel cell is disclosed where liquid water mist is introduced into the anode. A desiccant material directs the liquid water mist to the ion exchange membrane. Evaporation of a portion of both the product water and the supplied liquid water cools the cell and eliminates the need for separate cooling chamber.
U.S. Pat. No. 4,824,741 discloses a fuel cell utilizing a solid polymer electrolyte membrane cooled by evaporation of water in the hydrogen reactant chamber of the cells. A porous graphite plate or water permeated membrane is disposed in the hydrogen reactant chamber adjacent to the electrolyte membrane. If a graphite plate is used, it is preferably grooved on the surface facing the electrolyte. The resultant lands preferably contact the supported catalyst layer on the membrane to cool the latter. Water is forced into the pores of the plate or membrane from the edge thereof, and the water vapor is carried out of the cells in the hydrogen reactant exhaust stream.
U.S. Pat. No. 5,262,249 describes an internally cooled PEM fuel cell device. It includes a pair of substantially coextensive electrode components each of which includes a porous central region and a fluid impermeable peripheral region circumferentially completely surrounding the central region. It also includes a proton exchange membrane component interposed between at least the central regions of the electrode component. The fuel cell device further includes an arrangement for cooling the fuel cell, including at least one enclosed cooling channel situated at the peripheral region of one of the electrode components and supplied with fresh cooling medium, with the spent cooling medium being discharged from the cooling channel. There is further provided a heat transfer device that is operative to transfer heat from the central region to the peripheral region on the one electrode component.
All previously known and available evaporative cooling methods and designs for fuel cells have one common disadvantage in that the maximum cool temperature that may be reached is the wet bulb temperature of outside air, which cannot guarantee efficient rejection of heat from a fuel cell. This limited maximum of cooling that can occur has proven to be commercially disadvantageous to current fuel cell systems and apparatuses especially for PEM fuel cells. Because of the indirect and direct cooling, the herein invention yields a lower temperature that is below the wet bulb temperature and approaching the dew point temperature.
There are many methods and designs for humidifying fuel cells. For example, U.S. Pat. No. 5,382,478 discloses an electrochemical fuel cell stack, which has a humidifying section located upstream from the electrochemically active section. The inlet fuel and oxidant streams are introduced into the humidifying section without first being directed through the electrochemically active section. The upstream location of the humidification section in the stack enables the number of manifold opening in the active section to be reduced.
U.S. Pat. No. 5,432,020 describes a process and an apparatus humidifying the process gas for operating fuel cell systems. To ensure high efficiency, the process gas must be introduced at a predetermined temperature and humidity. A metered quantity of fine water droplets is injected into the gas supply line, by way of which the process air is humidified. If the fuel cell is operated under pressure, the process air generally has to be cooled after it has been compressed. The process air is automatically cooled as a result of a partial evaporation of the water droplets while the residual quantity of water in the form of droplets is introduced into the fuel cell.
It is desirable to have high humidification of the reactant gases in PEM fuel cells operating above about 60 degrees C. This has been confirmed by the general experience of PEM fuel cells users. However, this desirable feature can make it difficult to choose the optimum operating temperature for the fuel cells. The higher the temperature, the better the performance, mainly because the cathode overvoltage is reduced. However, once above 60 degrees C. the humidification problems increase, and the extra weight and cost of the humidification equipment can exceed the savings derived from having a smaller and lighter fuel cell. It is obviously desirable to run PEM fuel cells without externally humidifying the reactant gases as it reduces cost, size and complexity.
Buchi and Srinivasan have described the humidification process for PEM fuel cells (see Buchi F. N., Srinivasan S., (1997) xe2x80x9cOperating proton exchange membrane fuel cells without external humidifying of the reactant gases. Fundamental aspects.xe2x80x9d Journal of the Electrochemical Society, vol. 144, N8, pp2767-2772). There it is shown that even below 60 degrees C. the maximum power from a fuel cell is reduced by about 40% if no external humidification is used.
In small systems, this can be a price worth paying. One way of doing this is also described by Buchi and Srinivasan in which the oxygen and hydrogen flows are in opposite directions across the membrane electrode assembly. The water flow from anode to cathode is the same in all parts, as is the xe2x80x9celectro-osmotic dragxe2x80x9d, and is directly proportional to the current. The back diffusion from cathode to anode varies, but is compensated for by the gas circulation. Other aids to an even spread of humidity are narrow electrodes and thicker gas diffusion layers, which hold more water.
Engineers at Paul Scherrer Institute in Switzerland have demonstrated an external humidification system for fuel cells (see Kiefer J. et al xe2x80x9cRadiation grafting a versatile membrane preparation tool for fuel cell applicationsxe2x80x9d, Proceedings of the European Fuel Cell Forum Portable Fuel Cells conference, Lucerne, pp227-235, 1999). The warm damp air leaving the cell passes over one side of a membrane, where it is cooled. Some of the water condenses on the membrane. The liquid water passes through the membrane and is evaporated by the drier gas going into the cell on the other side. In this case, for example a 2.0 kW fuel cell, only the air is humidified. The membrane electrode assembly is particularly thin, and this permits the anode side to be hydrated by back diffusion.
Another approach for humidifying fuel cells is to directly inject liquid water into the fuel cell (for example, see U.S. Pat. No. 4,826,742). But this can lead to the electrode flooding and the cell ceasing to work.
All known and available humidifying methods and designs for fuel cells have one common drawback in that they normally cannot insure the necessary quantity of water molecules for hydrogen ion conductivity through the ion exchange membranes. All previously known evaporative humidification processes are not effective and have limited humidity corresponding to the wet bulb temperature of outside air. The present invention overcomes this drawback.
Fuel cells employing such prior membranes require water to be removed from the cathode (oxidant) side. There are many methods and designs for this purpose. For example, U.S. Pat. No. 4,826,741 describes an ion exchange fuel cell assembly with improved water management, where this fuel cell are characterized by a liquid permeable electricity-conductive member in the form of a layer for storage and transport of water produced in the fuel cell. The fluid permeable member is separated from the electrodes and the fuel cell is contained between outer impermeable sheets.
A method and an apparatus for removing water from electrochemical fuel cells are described in U.S. Pat. No. 5,441,819. In them, liquid water accumulated at the cathode can be removed by maintaining a partial pressure of water vapor in the hydrogen-containing gas supply below the saturation pressure of water vapor therein. Water accumulated at the cathode is drawn by a concentration gradient toward the anode across the membrane and is absorbed as water vapor into the hydrogen-containing gas supply between the inlet and the outlet.
U.S. Pat. No. 5,503,944 discloses a water management system for solid polymer electrolyte fuel cell. Water appearing on the cathode side of each cell membrane is pumped into the water circulation passages through the porous oxidant reactant flow field plates. A positive pressure drop created between the cathode reactant flow field of each cell and the coolant water circulation passages between each cell is used to pump the water.
Management of water migration within known fuel cell devices is inefficiently realized and has complicated designs. The product water must be drawn away from the cathode side of the cells, and makeup water must be provided to the anode side of the cells in amounts to prevent dryout, while avoiding flooding, of the anode side of the electrolyte membrane. Simultaneously it is necessary to reject heat from the cathode side of the electrolyte membrane.
Using previously known methods and designs, it is impossible to efficiently realize these various actions simultaneously. This in turn decreases productivity of the fuel cells. The present invention addresses this problem.
U.S. Pat. No. 6,106,964 discloses a method for humidifying and adjusting the temperature of a reactant gas stream supplied to PEM fuel cell. In it, an exhaust reactant stream from a fuel cell (for example, air) is used to heat and humidify an air stream supplied to the fuel cell. In this case, the exhaust reactant stream exiting the fuel cell will typically be warmer and have a higher partial pressure of water pressure than the supply reactant stream. The reactant gas supply streams for the PEM fuel cell include a fuel supply stream (for example, hydrogen) and an oxidant supply stream (for example, air) that are supplied to fuel and oxidant inlet ports of the fuel cell respectively. The PEM fuel cell also typically has both a fuel exhaust stream and an oxidant exhaust stream exiting the fuel cell via fuel and oxidant exhaust ports. However one of the reactants may be essentially dead-ended with optional intermittent venting of inert components. In this manner, water and heat in the conventional method are transferred from the reactant gas exhaust stream to the reactant gas supply stream across the water permeable membrane of the combined heat and humidity exchanger.
Known methods and systems (such as U.S. Pat. No. 6,106,964) for humidifying and adjusting the temperature of a reactant gas stream supplied to PEM fuel cells have major disadvantages. First, they cannot organize the efficient cooling process to reject heat from a fuel cell. Additionally, the methods are inefficient in providing the humidifying process for the reactant gas supply stream, because the evaporative humidifying processes are limited by the wet bulb temperature of outside air. Using known combinations of heat and humidity exchangers it is possible to get only one heated and moistened useful product, either a fuel supply stream or an oxidant supply stream of the fuel cell. But this is not enough for effective operation of a fuel cell. Usually a fuel cell also needs to use a coolant stream (air or water). In this connection, U.S. Pat. No. 6,106,964, employs an additional apparatus as a cooler for a coolant stream. Moreover, it is sometimes necessary (in addition to the cooling of a coolant) to humidify a fuel supply stream and an oxidant supply stream of the fuel cell simultaneously. Again, known methods cannot do it as they cannot perform the processes of humidifying and adjusting the temperature of a reactant stream and simultaneously cooling a conditioned space.
The new and novel fuel cell systems of the present invention with indirect and direct evaporative cooling combine the humidifying of reactant streams, adjusting the temperature of the reactant streams, and managing water and thus avoid the above-mentioned drawbacks.
The purpose of this invention is to increase the efficiency of operation of fuel cell systems, such as proton exchange membrane (PEM) fuel cells. In one embodiment, it is accomplished by more efficient processes of cooling of a coolant and humidifying a fuel supply stream and/or an oxidant supply stream of the fuel cell simultaneously, using only one additional inexpensive device. In another embodiment all the operative processes of cooling and humidifying are realized inside the system without any additional device.
In the first embodiment, an evaporative cooler is used. One example of this apparatus is described in detail in co-owned PCT Applications PCT/US01/04081 and PCT/US01/04082 filed in Feb. 7, 2001, which are incorporated herein by reference. Another example is the apparatus of co-owned PCT application PCT/US01/30468 filed Sep. 27, 2001, also incorporated herein by reference. This apparatus contains product channels for product fluids and dry and wet working channels with inlets and outlets for working fluids, for example air or gas. Product (air or any kind of gas or liquid) is cooled inside the product channels below the wet bulb temperature, approaching the dew point temperature without adding humidity to the product fluid.
The resultant cold and dry product air is directed to the cathode of a fuel cell. In this case, the cold and dry air performs two functions: first, it is an oxidant supply stream for the fuel cell and second, it is coolant for the fuel cell. Passing through the cathode of a fuel cell, this air actively absorbs heat, which is produced at the cathode when the hydrogen ions and oxygen combine. Simultaneously a fuel supply stream (for example, hydrogen) as a working gas is passing first into a dry working channel and later a wet working channel of the evaporative cooler and gains moisture for the hydrogen, thus aiding the hydrogen conductivity through the ion exchange membrane. After its exit from the apparatus, it is directed to the anode of a fuel cell. The absolute humidity of this resultant fuel supply stream is always more than by any other known methods of moistening. This aids the increase of hydrogen ion conductivity through the ion exchange membranes and consequently the efficiency of operation of the fuel cell system. Further efficiency occurs as the added moisture for the wet working channels of the apparatus is obtained by water product from the cathode of the fuel cell. The anions formed at the cathode react with hydrogen ions that have crossed the membrane to form water as the reaction product and thus is available. It is clean water, which does not create any kind of deposit build up on the surface of a fuel cell.
Sometimes an apparatus such as the above dew point indirect evaporative cooler can be used not only for supporting the operation of the fuel cell system, but also as an air conditioner for a conditioned space or a cabin of a vehicle at a separate location from the fuel cell. In this case cold product air, after its exit from an apparatus for dew point indirect evaporative cooling, is divided into two cold flows with one being directed as air reactant stream (oxidant) to the cathode of a fuel cell and the other to a consumer, for example, in a conditioned space or a cabin of a vehicle.
Sometimes there is a necessity to humidify only an oxidant supply stream. In such cases, outside air as an oxidant supply stream is directed into working channels and hot coolant is directed into product channels of the apparatus for dew point indirect evaporative cooling. Passing through these channels, moist air is directed to a cathode and cold coolant is used for rejection heat from a fuel cell or other uses.
The present invention can perform the humidifying processes for both reactant gas supply streams of a fuel cell (i.e. the oxidant supply and the fuel supply streams) simultaneously. To improve the action of the hydrogen ion conductivity through the ion exchange membranes by adding water molecules, the apparatus can cool coolant (when it is passing through the product channels) and humidify the oxidant supply stream as well as fuel supply stream simultaneously when they are passing through the separate working channels. The design of the apparatus for dew point indirect evaporative cooling allows separation of the reactant streams.
The design of the apparatus permits dividing not only the working channels for separate use and treatment of the reactant gas (hydrogen) and air (oxidant)) streams in them but also separate use of the product channels for coolant and outside air (oxidant) streams. In connection with this scheme it is possible simultaneously to organize the cooling process for outside air before its intake by an air compressor and its subsequent use as an oxidant stream for the fuel cell. Cooling of the air stream, before its intake by the air compressor, increases the density of air stream and decreases the consumption of energy by the compressor.
It is beneficial to have one apparatus in a single design which can simultaneously humidify the fuel supply and oxidant supply streams to cool coolant for the fuel cell and also to cool, for example, a conditioned space or a cabin of a vehicle.
In another embodiment, the fuel cell system has all the required operative processes of humidifying of the fuel supply and/or oxidant supply streams and absorbing heat from the fuel cell inside of the fuel cell system without any additional device. As a result this fuel cell system contains a solid polymer electrolyte membrane having anode and cathode catalyst layers and cathode and anode separator plates on opposite sides of a fuel cell, which are provided with air (oxidant) for the cathode and fuel reactant gas (hydrogen) for the anode. Each separator plate has a wet side wetted by water from a cathode, and opposing dry side separated by a waterproof or low permeability, heat transfer plate. Outside air as an oxidant stream is directed first across the dry side of the cathode separator plate and then to its wet side, where it flows, picks up evaporated water, and becomes humidified. This moist air stream is transferred to the condenser where the water is condensed out. Simultaneously a fuel supply stream such as a hydrogen stream is directed for cooling across the dry side of the anode separator plate cooled by the heat exchanger plate which separates the wet side and dry side of the anode separator. Hereafter one part of this cold stream is directed to the wet side of the anode separator plate, where this part of the stream flows and becomes humidified across the wet side by evaporation exiting outside. Another part of the cold hydrogen stream, after passing through the dry side of the anode separator plate, is transferred to the condenser for indirect contact with moist air stream from the cathode plate. As a result of this contact, water is condensed from the moist air stream and this air stream is then passed outside.
The cold part of the hydrogen stream, after passing through the condenser, is directed to the wet side of the anode plate where it takes the evaporated water and is passed outside. Water from the condenser is directed for wetting the wet side of the anode separator plate and (if it is necessary) the wet side of the cathode separator plate.
In some cases it is advantageous to organize another similar precooling process for the air reactant stream (oxidant), before passing it through the wet side of the cathode separator plate.
In another embodiment, the outside air and hydrogen streams are directed across respectively the cathode and anode dry sides of the respective separator plates. The air and hydrogen are permitted to flow through the plates from the dry sides to the wet sides of these plates, for example by using perforations in the heat exchanger plates. This action can help to reduce any pressure drop for the moving streams and also to increase the process of heat and mass exchange between the streams on both sides of the separator plate.
The described fuel cell system can thus be a source of electricity power while simultaneously an apparatus for indirect evaporative cooling. In this case outside air, after its passing and cooling across the dry side of the cathode separator plate, is divided into two flows. One flow is directed across the wet side of the cathode separator plate and then to the condenser. The other flow is directed to a consumer, for example, in a conditioned space or a cabin of a vehicle.